Map Migration

7/29/2019 Map Migration

1/19

14.1

Map Migration

Map

Migration

Map Migration

What is map migration?

What forms does it take?

When should we use it?


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14.2

Map Migration

Depth conversion takes three forms:

Vertical scaling, for flat lying strata (or when the seismic

data have been depth migrated and scaled back to time) -

this is what we have considered so far.

Image ray migration for 3D time migrated data.

Normal incidence ray migration for stacked

(unmigrated) data or 2D grids of time migrated data - after

demigration of the picks.

Ray migration can take place in 2D as event migration on aline-by-line basis, or in 3D as map migration.

Map Migration

Depthscaling

Macro-

velocity

model

Normal

incidence

migrationDepth maps

Image

ray

migration

Seismic

horizontimes

WellVelocity

Z.O. or

image ray

modelling

Compare

SeismicVelocities

Analyticfunctions

Velocity

Maps

Calibrate


3/19

14.3

Map Migration

Depth scaling works well in the presence of small structural

dips (less than 10) or when the seismic events are in their

correct lateral position from depth migration.

In the former case we need to know when depth scaling is

inadequate and event (2D) or map (3D) migration is required.

Depth Scaling

Map Migration

For 3D time migrated data the need for map migration is not

obvious since there are no misties in the data when we make

our interpretation. For such data image ray migration is

adequate for intermediate dips ( say 10 to 25).

For image ray maps, i.e. maps made from time migrated

sections, the need for map migration will become obvious in

the case of 2D data as misties manifest themselves in areasof steeper dip.

For 2D data the usual approach is to demigrate the

interpreted events, using migration velocities, in order to

create a normal incidence map which is then migrated.

Map Migration


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14.4

Map Migration

In the case of 2D data misties are caused by sideswipe.

Image ray migration will not overcome the problem, but

demigration of the 2D profiles to form a normal incidence

map, followed by map migration, will solve the problem.

Image ray lies within the

plane of the seismic

section.

Image ray does not

lie within the plane of

the seismic section.

Image Ray Bending

Seismic line

Reflector

Map Migration

Ray migration is a recursive process, i.e. we have to migrate

one layer at a time. The first layer, however, needs no

migration, the ray path is vertical. it is sufficient to depth scale

the image ray structure-in-time map.

We have to:

determine the direction of the image ray in the second

layer, according to Snells law, ray trace to find where it intersects the second horizon,

depth convert along the ray path and then remap the

second horizon.

The process is then repeated for horizon three, and so on.

Image Ray Migration


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14.5

Map Migration

We can estimate the need for map migration:-

Lateral positioning error is estimatedby:

xm - x t2Sin1(V1 - V2)(1 + 3Tan2)

Where

Tan1 V1dt1/dx1Tan VRMSdt2/dx2

For multiple layers:

Find 1 Tan-1( VA1dt1/dx1 )

1i = angle of incidence

Find angle of refraction 1r (Sin-1(V2Sin 1i /V1)

Displacement at 2nd layer V2t2tan(1r- 1i )

Find 2 Tan-1

( VA2dt2/dx2 )Angle of incidence 2i 2 + (1r- 1i )

Find angle of refraction 2r (Sin-1(V3Sin 2i /V2)

Etc.

x

t

2

1

V1

V2

xmx

t2

dt2

/dx2

dt1/dx1

Image Ray Migration

Map Migration

The need for image ray map migration :-

Two seismic

lines with their tie

positions

marked.

Data courtesy of Amoco

Image Ray Migration


6/19

14.6

Map Migration

The need for image ray map migration :-

The two ties made

vertically showing

the misties.

One section is

pivoted about the tie

position until the

dipping horizons of

interest tie.


Image Ray Migration

Map Migration


Approximating map

migration:

One section is pivoted about

the tie position until the

dipping horizons of interest

tie.

The lateral displacement isannotated on the map.

The procedure is repeated

on the other line

Image Ray Migration


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14.7

Map Migration

The two measured displacements are the approximatecomponents of the Map Migration vector: -

Surface line

positionsDisplacement

Displacement

Approximate

Map Migration

Vector

Image Ray Migration

Map Migration

Do exercise 14.1 on the next page.

Image Ray Migration


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Map Migration

The seismic lines on the next two sheets tie at the location marked by the heavy line.

Use the technique described earlier to investigate the approximate map migration

vectors at different seismic reflection times.

Draw a series of sketches below to illustrate the vectors at the different depths.

Explain your observations.

Exercise 14.1

14.8


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Map MigrationExercise 14.1

14.9


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Map Migration

14.10

Exercise 14.1

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Map Migration

14.11

Exercise 14.1


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Map Migration

14.12

Exercise 14.1

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13/19

14.13

Map Migration

The need for image ray

migration is greatest where

we have a structure beneath

a strongly dipping

overburden which contains

high velocity contrasts.

1 km

2 km

1 km

2900

2600

4350

4700

5400

5200

5000

51004550Example after Larner et al, Depth migration of imaged

time sections, Geophysics, May 1981

Image Ray Migration

Map Migration

Migrated depth map

with displacement

vectors and the

before and after

fault positions.

From a Paradigm Geophysical brochure.

Image Ray Migration


14/19

14.14

Map Migration

For normal incidence maps we can estimate the effects ofmigration as follows:-

x

z

dx

dz

t

Depth error

dz = dT / VRMSwhere dT is the vertical time

displacement given by

dT = T{1 - [1 - (VRMS2.tan2)/4]1/2}.

The horizontal displacement is

dx = (VRMS2T.tan)/4,

tan = T / x (time dip) from theunmigrated section,

T the two way time to the reflector.

Normal Incidence Migration

See Chun and Jacewitz 1981 for full details

Map Migration

As with image ray migration, normal incidence ray migration

is a recursive process, but this time, we need to migrate the

first horizon.

Lasthorizon?No

Time

picks

Velocities

Emergence

angle for

horizon

Time dip

for horizon

Migrate to

depth by

ray tracing

Next layer

Final model Yes



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14.15

Map Migration

The rays are traced into a

layer for a distance given by

L = To x velocity / 2

starting at an emergence

angle given by

sin = time dip x Ve/ 2.

The local time dip of the

event is given by

tan = t / x

Ve is the velocity with whichthe ray emerges from the

layer. T

XV1

Vi = V0 + Kz

VO

t

xTo

To


Map Migration

Migrated depth map with

displacement vectors and

the before and after fault

positions.

From Sattlegger GmbH brochure.



16/19

14.16

Map Migration

Ray Migration

Image ray migration is most sensitive

to errors in velocity, and hence errors

in angles.

Normal incidence migration is most

sensitive to the emergence angle

which depends on velocity gradient

and surface dip.

Map Migration

Summary

Vertical scaling is adequate when dips are less than 10.

Image ray map migration is adequate for dips between about

10 and 25 with 3D time migrated data.

For 2D time migrated data with dips over 10 and 3D time

migrated data with dips over ~25 use normal incidence map

migration.


17/19

14.17

Map Migration

Summary

StabilityDip

Handling

HighLow ( 20)

NIRay

Migration

Map Migration

Map Migration

Integration

Analyticfunctions

/ Mapping

Depthscaling

Average,Interval,

InstantaneousMacro-

velocity

model

Image Ray

migration Depth maps

NI Raymigration

Seismic

horizon

times

Velocity log

Sonic logCheckshot

or VSP

Z.O. or

image ray

modelling

Compare

Seismic

Velocities

Dip,

Interpolation

Invert

(Dix/Bias)

Edit,

Smooth

Calibrate


18/19

14.18

Map Migration

Summary

Stack

Conventional

Processing

Time

Migration

Stacking /

Time migration

Velocity

Calibrated

Velocities

Horizon

Picks

Well

Data

Macrovelocity

Model

Depth

Scale / MapMigrate

Forward

Model

Map Migration


19/19

Map Migration

Exercise: compute the image ray bending.

In the following model the reflection times and time dips are observed on a time

migrated dip section at the flank of a salt dome and the interval velocities come

from a nearby well. You do not have map migration software available.

Should you consider map migration to depth convert the 4th event (the target

horizon)?

Hint: Find the average velocity down to each event, convert the time dips into

structural dips and then compute the lateral displacement of the image ray at

each interface.

0

720

1145

1315

1870

TWT msec

VI = 6760 ft/s

VI = 7246 ft/s

VI = 8130 ft/s

VI = 8889 ft/s

time dip = 400 msec over 280 traces




trace spacing = 75 ft.

Exercise 14.2

14 19

Documents

Map Migration